The invention relates to the field of hydrodynamic bearing assemblies, especially of the type that co-operate with a high speed spindle design. More specifically, the invention relates to apparatus for maintaining the bearing stiffness in a hydrodynamic bearing assembly despite substantial variations in temperature.
Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle. The information is accessed by means of read/write heads generally located on a pivoting arm that moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor that drives a hub to support the discs for rotation. Such motors, which are commonly referred to as spindle motors have a spindle or sleeve mounted by means of two ball or one or more hydrodynamic bearing systems to a motor shaft disposed in the center of the hub. Generally, such motors include a stator comprising a plurality of teeth arranged in a circle. Each of the teeth support a plurality of coils or windings that may be sequentially energized to polarize the stator. A plurality of permanent magnets are disposed in alternating polarity adjacent the stators. As the coils disposed on the stators are sequentially energized in alternating polarity, the magnetic attraction and repulsion of each stator to the adjacent magnets causes the spindle or hub to rotate, thereby rotating the disc and passing the information storage tracks underneath the head. This arrangement of stator teeth or laminations, coils and magnets may be mounted either inside the hub or spindle, or lower on the shaft below the hub or spindle in order to make the cross-sectional area occupied by the spindle or rotating sleeve as small as possible.
The use of hydrodynamic bearing assemblies in such disc drive systems has become preferred due to desirable reductions in drive size and noise generation as compared to conventional ball bearing drive systems. In hydrodynamic bearings, a lubricating fluid, such as oil or even air, functions as the bearing surface between a base or housing or shaft and a rotating spindle or hub (or vice versa). The lubricating fluids require small gaps between the stationary and rotating members in order to provide the support, stiffness and lubricity required for proper bearing operation. Thus the drive components and assemblies require tight tolerances and precision assembly methods. Such demanding tolerance and assembly control could result in increased part and assembly costs, along with an increased level of quality control to ensure proper drive operation. Further, with temperature fluctuations during use of the drive, the gaps which are an essential part of each hydrodynamic bearing could easily vary. Any disc drive assembly, and therefore the hydrodynamic bearings which provide the required rotational support, must be capable of operating over wide temperature fluctuations.
Therefore, the problem presented is to provide a disc drive assembly comprising a hydrodynamic bearing, in which the assembly method is simple and straightforward, and results in a hydrodynamic bearing assembly which is easily repeatable and tolerant of wide fluctuations in temperature during operation.
The present invention has as its objective to provide a bearing design which enables bearing stiffness in a hydrodynamic bearing system to be reliably maintained.
A related objective of the invention is to provide a simplified design for a hydrodynamic bearing comprising a shaft and surrounding sleeve wherever the potential for temperature changes causing viscosity changes causing the hydrodynamic bearing stiffness to change beyond tolerable levels of the disc drive.
In summary, according to the present invention a hydrodynamic bearing is provided comprising a shaft having a pair of spaced apart conical bearings supporting a surrounding sleeve for rotations. The sleeve comprises a single piece rather than the two-piece design separated by a rubber expansion joint of the prior art. Instead upper and lower slots are cut into the sleeve, and the rings are pressed or otherwise lightly fitted into each slot. The single piece sleeve which also incorporates the bearing seats for the conical bearings on the shaft, is made of aluminum, while each ring is made of steel (as is the shaft and bearing cones). As the temperature rises at which the hydrodynamic bearing is operated, the sleeve expands axially putting the bearing seat closer to the cone, effectively compensating for the thinning of the fluid which supports the bearing seat and sleeve for rotation relative to the cone. Meanwhile, the steel ring, being of the same material as the shaft, prevents the aluminum sleeve from expanding radially away from the cone. Further, the axial length of the interface between the sleeve and ring can be adjusted to control the amount of thermal compensation into any system, allowing for very accurate control of the thermal compensations scheme.
Other feature and advantages of the will be apparent to a person skilled in this field who reads the following description of a exemplary preferred embodiment given in conjunction with the following drawings.